1. Field of Invention
The present invention relates to the milling of product for use as a sorbent in a pneumatic conveying system. More specifically, the invention relates to an in-line system for milling (see DEFINITIONS section), and methods of operation for milling that support the unique aspects of the system.
2. Description of Prior Art
The market for air pollution control (APC) technology has grown tremendously since the Environmental Protection Agency (EPA) began to enforce more stringent standards via the Clean Air Act and its multiple rounds of amendments (most recently in 1990). According to BCC Research, “the total global air pollution mitigation market was worth $59.3 billion in 2006, and will reach $83.5 billion in 2007. By 2012, it is estimated that the global market will be worth over $138 billion, a compound annual growth rate (CAGR) of 10.6%.” Thus, air pollution control devices have become nearly a ubiquitous component of industrial facility design and operation, and technology improvements are constantly needed in this area.
Specific pollutants such as SOx and NOx which contribute to haze, acid rain, and ozone issues have become the focus of the Clean Air Interstate Rule (CAIR). State-enforced standards have been developed based on federal requirements defining: (i) reasonably available control technology (RACT); and (ii) maximum achievable control technology (MACT) standards. Coal-fired power plants, as well as industrial/institutional boilers and other fossil fuel fired systems, contribute a large volume of SOx and NOx emissions, if not properly controlled. Although flue gas desulphurization (FGD) is effective in controlling SO2 to some degree, it is less effective upon SO3 emissions, which create a visible haze via stack emissions from the plant. An APC technology that can control both SO2 and SO3 emissions effectively is needed.
Cost is another issue in reducing emissions from coal-fired power plants. Many existing plants have already been retrofitted with very expensive FGD systems. Further capital cost to meet the standards and remove the SO3 haze and gases is prohibitive for many plants. What is needed is a lower cost solution to achieve the additional reduction in SO3 emissions and remove the stack emission haze and gases.
It has been found that injection of chemical sorbents, sulfur-absorbing materials, into the flue gas duct is an effective approach to reduce both SO2 and SO3 emissions from coal-fired power plants. Sodium bicarbonate, and hydrated lime can be used to absorb the gaseous sulfur and reduce SOx stack emissions. However, many sorbents are prone to binding and agglomeration making them difficult to work with. Another material found to be quite effective as an injected sorbent is trona/sodium bicarbonate, a naturally occurring sodium sesquicarbonate product. Trona/sodium bicarbonate and sodium bicarbonate can be quite expensive, but if the material is milled to a smaller particle size, the material becomes more cost-effective and more efficient in the process (milling the sorbent creates greater surface area for absorption). However, there are two technical challenges of milled trona/sodium bicarbonate or sodium bicarbonate. First, trona/sodium bicarbonate/sodium bicarbonate suppliers provide a wide variety of particle size in their specific product. This makes it difficult for a single duct injection system to accommodate multiple trona/sodium bicarbonate sources, and some particle diameter ranges may not even be usable in the system. Second, if the trona/sodium bicarbonate is pre-milled by the supplier, it is more difficult to handle and does not flow properly. Additionally, trona/sodium bicarbonate can agglomerate and clog delivery and piping systems. What is needed is a solution that accommodates a variety of trona/sodium bicarbonate particle sizes, and in addition does not cause additional problems with transport and injection of a milled trona/sodium bicarbonate product.
Some sorbent materials (see DEFINITIONS section) that have been proposed and/or used conventionally include: (i) sodium bicarbonate; (ii) dry hydrated lime; (iii) carbon; and/or (iv) trona.